This invention relates to a screw-type bone implant with a shaft, which is designed at one extremity to receive a dental prosthesis, which ends freely at its other extremity, and which has a threaded helix between the two extremities.
Bone implants for screwing into the jawbone onto which is subsequently mounted a dental prosthesis have been known for some time in a multiplicity of designs.
It is clear that a bone implant must be long-lasting and, accordingly, must support the dental prosthesis for as long a period as possible. In order to attain this goal, new proposals have continuously been submitted. From the abundance of examples, reference is made to German Patent Application (Offenlegungschift) No. 2,255,916, No. 2,540,077, and No. 2,628,443, which all disclose bone implants in the form of screw-type devices which consist in part of ceramics and in part of metal, and which are subdivided into a threaded section and a fastening section.
Despite the abundance of attempts toward creating a permanent bone implant, the results have not been satisfactory in all respects. It is therefore an object of this invention to provide a screw-type bone implant which represents an improvement over the prior art, primarily through greater sturdiness in the implanted condition.
This invention provides a screw-type implant of the originally mentioned type which solves this task, and in which the length of the shaft is greater by at least the axial length of the head which receives the dental prosthesis than the height of the jawbone viewed in the direction into which the implant is made. There are V-shaped cutouts in the threads of the threaded helix at distances along the arc of the helix which are smaller than 360 degrees and which are other than 180 degrees. The cutouts extend from the circumference of the helix and have tips which are positioned in the area of the shaft surface. Each cutout has two sides forming two contact surfaces which lie substantially in the longitudinal direction of the shaft. The contact surface having the greater distance from the free shaft end is provided with at least one protruding sharp edge extending in the direction of the thread. The external diameter of the threaded helix increases in axial direction from the free end of the shaft to a large final value.
It is advantageous in this instance to provide the free end of the shaft with the shape of a pointed cone.
Whereas, to implant the known screw-type implant it was necessary to initially pre-drill a hole having an external diameter almost equal to the external diameter of the implant threading, to implant the implant according to this invention it is only necessary to pre-drill a hole having the diameter of the shaft, inasmuch as the implant of this invention will cut the threading by itself.
The known screw-type device for implants could be kept in place only when bone matter had grown in and, immediately after implantation, a comparatively large hollow space remained from the small threading region radially inwardly toward the shaft, which space was only gradually filled in with growing bone substance. The known implants, therefore, could be initially subjected to only comparatively minor stresses. In addition, the initial, less-than-optimum anchoring of the implant in the spongelike substance of the bone resulted in the phenomenon that the implant could move, not only axially but also radially, when stresses were applied. This movement would naturally tend to impede the growing-in process and favor formation of soft bone replacement tissue or granulation tissue.
With the implant according to this invention, these difficulties no longer occur because no hollow spaces have to be filled in before the implant can be exposed to stress, and primarily also because of the length of the implant, so that when it is screwed into the jawbone, the implant is supported at both ends within the bone, i.e., at the entrance and at its free end, so that the implant supports itself after penetration of the spongelike substance on the compact bone substance which lies opposite the entrance. Thus, from the very beginning, axial as well as radial movement of the implant is practically impossible, whereby the initial healing process can take place without interference, and whereby durability is significantly enhanced.
A further consequence of the self-cutting capability of the bone implant according to this invention is that a portion of the bone substance need not be permanently removed. The implant can be squeezed into existing hollow spaces of the spongelike bone substance, and these portions of the bone substance can be better supplied with blood because of the lessened trauma. Thus, weakening of the jawbone is reduced.
In summary, the teaching of the invention provides that only a minimum of bone substance must be removed, namely, the volume of the implant shaft, while the threads completely and automatically cut their own path. Thus, the finished implant, from the very beginning, is embedded more evenly and more firmly in the jawbone than was hitherto possible. The formation of new bone substance, which is less able to withstand pressure than the original bone substance, will no longer occur because no empty cavities remain.